Carbonyl groups polyethylene

Oxidation of polyethylene with the formation of carbonyl groups can lead to a serious increase in power factor. Antioxidants are incorporated into compounds for electrical applications in order to reduce the effect. 

Papirer et al. used ATR, XPS, and SIMS to determine the effect of flame treatment on adhesion of polyethylene and polypropylene to styrene/butadiene (SBR) rubber [8]. Each flame treatment consisted of a 75-ms pass over a circular burner. The distance between the upper flame front and the polymer was kept fixed al 8 mm. A band was observed near 1720 cm" in the ATR spectra and assigned to carbonyl groups this band increased in intensity as the number of flame... 

Friedrich et al. also used XPS to investigate the mechanisms responsible for adhesion between evaporated metal films and polymer substrates [28]. They suggested that the products formed at the metal/polymer interface were determined by redox reactions occurring between the metal and polymer. In particular, it was shown that carbonyl groups in polymers could react with chromium. Thus, a layer of chromium that was 0.4 nm in thickness decreased the carbonyl content on the surface of polyethylene terephthalate (PET) or polymethylmethacrylate (PMMA) by about 8% but decreased the carbonyl content on the surface of polycarbonate (PC) by 77%. The C(ls) and 0(ls) spectra of PC before and after evaporation of chromium onto the surface are shown in Fig. 22. Before evaporation of chromium, the C(ls) spectra consisted of two components near 284.6 eV that were assigned to carbon atoms in the benzene rings and in the methyl groups. Two additional... 

Figure 14 shows the ATR spectrum of the etched polyethylene surface treated with a chronic acid group [76]. Absorption bands due to surface treatment appear at 3300, 1700, 1260, 1215, and 1050 cm". The band at 3300 cm represents the absorption due to the hydroxyl group and that at 1700 cm " is due to the carbonyl group. The bands at 1260, 1215, and 1050 cm are all due to the alkyl sulfonate group. 

Formation of carbonyl groups which may occur when polyethylene is overheated, such as in an extrusion coating operation. Here a proper balance between extrudate temperature and exposure time to air is required. 

The radical-promoted reaction between polyethylene and hexafluoro-acetone is shown in Equation 1. It had been demonstrated previously in the case of simple hydrocarbons (8) that the addition of a carbon radical to the carbonyl group of hexafluoroacetone can take place in two modes, to yield the product, of substitution with either a fluoro-... 

Allyl Free Radicals. Ayscough and Evans (3) have recently studied, by ESR measurements, the types of allylic free radicals produced by gamma-irradiation of several monomeric olefins. In irradiated polyethylene the allyl free radical is quite stable, persisting for several months at room temperature (31). The presence of these allyl free radicals is most noticeable in the case of high density polyethylene, and this type of free radical is undoubtedly the cause of the slow oxidation of polyethylene at room temperature, which lasts for 40 or more days after irradiation (10). Williams and Dole (40) could observe little if any oxidation of low density polyethylene when it was exposed to air after irradiation. By oxidation we mean formation of carbonyl groups as detected by infrared absorption studies at 1725 cm"1. Parenthetically, it should be noted that adding an oxygen. molecule to a free radical produces initially another type of free radical, a peroxy free radical, but in this paper we shall not discuss free radicals of this or any other types except those of hydrocarbons. 

Figure 1. Logarithm of the phosphorescence intensity IT as a function of reciprocal temperature for films of (a) polyethylene and (b) polystyrene containing traces of carbonyl groups.

The effects of irradiation damage on polymers has also been followed using FT-IR 219,235,236), by obtaining difference spectra after specific exposure durations in a variety of environments. For polyethylene 219), the difference spectra revealed an increase in the vinyl end groups and an increase of ketonic carbonyl and trans-vinylidene double bonds. It was found that for every free radical formed approximately 10 carbonyl groups and 2 carbon-carbon double bonds were formed 236 ... 

The formation of oxygen anion radicals and molecules of ozone also should be counted with at the ionization initiation of oxidation [26]. Initiation reaction caused by oxygen anion radicals may play an important role within the polymer bulk while the effect of ozone forming in the surrounding air atmosphere will include only the formation of radicals on the polymer surface. The latent effect of ionization initiation on polymer oxidation which is very distinct may be documented on a relatively fast increase of concentration of carbonyl groups, observed over 1 year after irradiation crosslinking of polyethylene [27]. 

LDPE samples were crosslinked by e-beam irradiation under inert gas conditions and then extracted exhaustively with m-xylene. Prior to photosulfonation, the cross-linked polyethylene samples were also analyzed with respect to the identity and purity by FTIR-spectroscopy. The FTIR spectrum (see Fig. 9) is virtually identical with that of not cross-linked LDPE and no oxidation products (e.g. carbonyl groups, 1720 cm-1) could be detected. 

This strategy also allowed the one pot preparation of polyethylene with regularly placed carbonyl groups, according to the following Scheme 35 [79]. 

For example, with only 1% of the carbonyl group, the G value for crosslinking of polyethylene was doubled. Furthermore, as shown in Table TV there was a concurrent reduction in the... 

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